This invention relates to a process for recovering phosphate values from a finely divided ore containing the phosphate values and gangue by froth flotation.
Flotation is a process of treating a mixture of finely divided mineral solids, e.g. a pulverulent or finely ground ore, suspended in a liquid whereby a portion of such solids is separated from other finely divided solids, e.g. clays and other like materials present in the ore by introducing a gas (or providing a gas in situ) in the liquid to produce a frothy mass containing certain of the solids on the top of the liquid and leaving suspended (unfrothed) the other solid components of the ore. Flotation is based on the principle that introducing a gas into a liquid containing particles of different materials suspended therein causes adherence of some gas to certain suspended solids and not to others and makes the particles having the gas thus adhered thereto lighter than the liquid. Accordingly, these particles rise to the top of the liquid to form a froth.
Agents known as collectors are used in conjunction with flotation to promote recovery of the desired material. These agents have the ability of selectively attaching themselves to the particles of the desired material and improving the affinity of those particles for the gas bubbles.
Froth flotation is the main method used in South Africa for separating phosphates from its associated gangue. The collector which is invariably used is a fatty acid, particularly tall oil fatty acid. Typically recoveries of phosphate of the order of 60% are achieved using this known method.
USSR Pat. No. 688,235 (Chemical Abstracts Vol. 91,1979 195331t) discloses a process whereby phosphate containing minerals are floated from silicate-carbonate-type ores with high dolomite content with a fatty acid collector in an alkaline medium, conditioned with gelatinized starch. The process is improved using a mixture of the fatty acid collector with an alkyl hydroxamate, hydroxyethylated alkyl phenols, and nonpolar oil in the proportion of 0.75-1.25: 1.5-2.5: 0.75-1.25: 0.75-1.25.
Swedish Pat. No. 326,417 (Chemical Abstracts Vol. 77,1972 64149c) discloses that the selectivity for separating from apatite by flotation is considerably increased in an acidic medium. The mineral is treated with a hydrophobizing collector and then flotation is carried out at an initial pH of 2-4, which is allowed to increase to 6-7 during flotation. The hydrophobizing agent contains saturated or unsaturated C.sub.8-24 fatty acids. Advantageously, a surfactant (oil soluble alkylbenzenesulfonates, alkylsulfates, or nonionics) is added as an emulsifier, and pine acid, tar acid or iso alcohol as a frother. This mehod can be combined with other flotation methods by which silicates and silica are removed before the calcite by using the same collector in an alkaline medium. In an example, 100 g of a phosphate mineral (particle size 40-50 .mu.m) containing approximately 20% P.sub.2 O.sub.5 and calcite impurities not removable by alkaline flotation was conditioned into a 70% pulp in the presence of 2500 g of tall oil fatty acid per ton of mineral. This was added as a 1% emulsion with the aid of 5% of a 1:16 nonylphenol-ethylene oxide condensate.
Sufficient water was added to give a pulp weighing 3200 g and the system was acidified with dilute HC1 to a pH of 2.5. After addition of 40 ppm 1:10 nonylphenol-ethylene oxide condensate as a frother, flotation for 30 minutes yielded 75.3% residue containing 24.0% P.sub.2 O.sub.5.
Although various methods for the flotation of phosphate values from a phosphate containing ore are known, new processes are always needed. This is particularly so because the complexity and diversity of the interactions that can occur between reagents and minerals during the flotation process are such that understanding of this subject is still very inadequate. In practise virtually every mineral deposit requires its own unique flotation circuit and reagent suite.